Factor out duplication of code responsible for constructing and disposing of objects.
Factor out common code from Fixie.Samples test classes.
Add delegate type Factory, used by both CreateInstancePerCase and CreateInstancePerFixture to create the fixture instances.
When specifying whether a Convention should create an instance per case vs. an instance per fixture, allow regular construction to be replaced by an arbitrary factory method.  Fixie.Samples gains an example convention using this feature, so that fixture classes can take part in IoC.